The present invention relates generally to the field of electrophotography and in particular to a method of adjusting a white vector by partial exposure of selected white image areas of the latent image on a photoconductive unit.
The basic electrophotographic process is well known in the art, and described briefly with reference to FIG. 1. FIG. 1 is a schematic diagram illustrating an exemplary image forming unit 10 (for the purpose of this description, only the solid-line elements of FIG. 1 are considered). Each image forming unit 10 includes a photoconductive unit 12, a charging unit 14, an optical unit 16, a developer roller 18, a transfer device 20, and a cleaning blade 22.
In the embodiment depicted, the photoconductive unit 12 is cylindrically shaped and illustrated in cross section. However, it will be apparent to those skilled in the art that the photoconductive unit 12 may comprise any appropriate shape or structure. The charging unit 14 charges the surface of the photoconductive unit 12 to a uniform potential, approximately −1000 volts in the embodiment depicted. A laser beam 24 from a laser source 26, such as a laser diode, in the optical unit 16 selectively discharges discrete areas 28 on the photoconductive unit 12 that are to be developed by toner (also referred to herein as “pels”), to form a latent image on the surface of the photoconductive unit 12. The optical energy of the laser beam 24 selectively discharges the surface of the photoconductive unit 12 to a potential of approximately −300 volts in the embodiment depicted (approximately −100 volts over the photoconductive core voltage of −200 volts in this particular embodiment). Areas of the latent image not to be developed by toner (also referred to herein as “white” or “background” image areas), indicated generally by the numeral 30, retain the potential induced by the charging unit 14, e.g., approximately −1000 volts in the embodiment depicted.
The latent image thus formed on the photoconductive unit 12 is then developed with toner from the developer roller 18, on which is adhered a thin layer of toner 32. The developer roller 18 is biased to a predetermined voltage intermediate to the voltage of the latent image areas to be developed and the latent image areas not to be developed, such as approximately −600 volts in the embodiment depicted. Negatively charged toner 32 is attracted to the more-positive discharged areas 28, or pels, on the surface of the photoconductive unit 12 (i.e., −300V vs. −600V). The toner 32 is repelled from the less-positive, non-discharged areas 30, or white image areas, on the surface of the photoconductive unit 12 (i.e., −1000V vs. −600V), and consequently the toner 32 does not adhere to these areas. As well known in the art, the photoconductive unit 12, developer roller 18 and toner 32 may alternatively be charged to positive voltages.
In this manner, the latent image on the photoconductive unit 12 is developed by toner 32, which is subsequently transferred to a media sheet 34 by the positive voltage of the transfer device 30, approximately +1000V in the embodiment depicted. Alternatively, the toner 32 developing an image on the photoconductive unit 12 may be transferred to an Intermediate Transfer Mechanism (ITM) such as a belt 38 (see FIG. 3), and subsequently transferred to a media sheet 34. The cleaning blade 22 then removes any remaining toner from the photoconductive unit 12, and the photoconductive unit 12 is again charged to a uniform level by the charging device 14.
The above description relates to an exemplary image forming unit 10. In any given application, the precise arrangement of components, voltages, and the like may vary as desired or required. As known in the art, an electrophotographic image forming device may include a single image forming unit 10 (generally developing images with black toner), or may include a plurality of image forming units 10, each developing a different color plane separation of a composite image with a different color of toner (generally yellow, cyan and magenta, and optionally also black).
Additionally, in the above description, the toner 32 is dry, and toner particles adhere directly to the developer roller 18 and pels of the photoconductive unit 12. As known in the art, in another embodiment, the toner may comprise a liquid medium in which electrically charged, pigmented toner particles are suspended. One or more colors of liquid toner may be successively applied to the developer roller 18 by an appropriate fluid delivery mechanism (not shown), with each color of toner selectively removed from the developer roller 18 following development of the associated image color plane on the photoconductive unit 12. Alternatively, the image forming device may include a plurality of image forming units 10, each such unit 10 applying a different color liquid toner. The liquid toner develops the latent image on the photoconductive unit 12, and the developed image is transferred to an ITM 36 or a media sheet 34, as described above. Additional steps such as drying, cleaning, liquid removal and recovery and the like may be required, as known in the art. The present invention is not limited to dry toner 32, and liquid toner based image forming devices are within its scope.
The difference in potential between non-discharged areas 30 on the surface of the photoconductive unit 12—that is, white image areas or areas not to be developed by toner—and the surface potential of the developer roller 18 is known as the “white vector”. This potential difference (with the white image areas 30 on the surface of the photoconductive unit 12 being less positive than the surface of the developer roller 18) provides an electro-static barrier to the development of negatively charged toner 32 on the white image areas 30 of the latent image on the photoconductive unit 12. A sufficiently high white vector is necessary to prevent toner development in white image areas; however, research indicates that an overly large white vector detrimentally affects the formation of fine image features, such as small dots and lines. In exemplary embodiments of image forming devices, a white vector of 200-250V results in acceptable image quality while preventing toner development in white image areas.
The optimal white vector for each image forming unit 10 within an image forming device may be different, due to differing toner formulations, component variation, difference in age or past usage levels of various components, and the like. One way to achieve a different white vector at each image forming unit 10 is to power each charging device 14 to the desired non-discharged potential (e.g., the potential of the corresponding developer roller 18 plus the desired white vector). This would generally require a separate power supply for charging the photoconductive unit 12 in each image forming unit 10, increasing the image forming device cost and weight, reducing reliability, and precluding a compact design, as each power supply requires space.